The products of acid-catalyzed reactions of olefins may include primarily olefins from straight oligomerization or mixtures of olefins, paraffins, cycloalkanes and aromatics. The product spectrum is influenced by both reaction conditions and the nature of the catalyst.
The oligomerization of olefins over zeolite catalysts is influenced by many factors; including thermodynamics, kinetic and diffusional limitations, shape-selectivity and side reactions.
Molecular weight growth occurs by condensation of any two olefins to a single higher olefin. The acid-catalysed oligomerization of the olefins occurs via a carbocationic mechanism as shown in the example below:

Carbocation 1 can undergo hydride and methyl shifts or it can lead to the formation of trimers via addition of Carbocation 1 to a monomer.
Olefins also undergo double bond and skeletal isomerization. In addition to oligomerization, any two olefins may react to disproportionate to two olefins of two different carbon numbers. Yielding intermediate or “nonoligomer” olefins, this will tend to randomize the molecular weight distribution of the product without significantly changing its average carbon number. Olefin cracking may also occur simultaneously with oligomerization and disproportionation. In practice, the kinetics of the oligomerization, disproportionation and cracking reactions determines the olefin product distribution under process conditions. Olefins may also undergo cyclization and hydrogen transfer reactions leading to the formation of cycloolefins, alkyl aromatics and paraffins, in what has been termed conjunct polymerization.
Thermodynamics dictate that at high temperature or low pressure, the distribution is centred in the light olefin range whereas at low temperature and high pressure, it tends to favour higher molecular weight olefins. At low temperature, mostly pure oligomers are formed with the majority of the product being trimer and tetramer. With increasing temperature, more disproportionation and cracking and, hence, randomization of the olefin distribution occur. At moderate temperatures, the product is essentially random and average carbon number is maximised.
The reactivity of olefins decreases with increasing carbon number due to the diffusional limitations within the pore system and the lower probability of coincident reaction centers of the molecules for a bimolecular reaction.
The ignition performance of diesel fuel represents an important criterion, similar to the octane quality of gasoline. The ignition performance of a diesel fuel, described by the cetane number, is determined by its composition and behaves opposite to octane quality. Hydrocarbons with high octane number have a low cetane number and vice versa.
The cetane, like octane number, is determined by comparative measurements. Mixtures of a-methylnaphthalene with very low ignition quality (cetane number of 0) and cetane (n-hexadecane) with very high ignition quality (cetane number of 100) are used as references. The cetane number of a reference mixture is given by the volume percentage of cetane in a-methylnaphthalene.
A high cetane number is advantageous for the ignition and starting behaviour, the reduction of white and black smoke and noise emission.
None of the classes of substances present in diesel fuel fulfils all the criteria equally well; for example, n-paraffins, which have a very good ignition performance and low smoking tendency, show poor low-temperature behaviour. See Table A below:
TABLE AProperties of hydrocarbon groups with regard to theirsuitability for diesel.Cold FlowSmokingCetane no.PropertiesDensityTendencyn-ParaffinsGoodPoorLowLowIsoparaffinsLowGoodLowLowOlefinsLowGoodLowModerateNaphthenesModerateGoodModerateModerateAromaticsPoorModerateHighHighDensity
The density of a diesel fuel has also a considerable effect on the engine performance. Because the quantity of fuel injected into an engine is metered by volume, the mass of fuel introduced into the engine increases with density. A higher fuel density leads to an enrichment of the fuel—air mixture which in principle, yields a higher engine power output; at the same time, however, negative effects on exhaust gas emissions occur.
Sulphur Content
Exhaust gas emissions are also affected by the sulphur content of diesel fuel. In addition, acidic combustion products arising from sulphur can lead to engine corrosion.
Viscosity
For optimal performance, the viscosity of a diesel fuel must lie between narrow limits. Too low a viscosity can lead to wear in the injection pump; too high a viscosity deteriorates injection and mixture formation.
Cold Flow Properties
The composition of diesel fuel also affects its filterability at low temperatures to a great degree. Particularly, n-paraffins with high ignition quality, tend to form wax crystals at low temperatures, which can lead to clogging of the fuel filter. The cloud point and cold filter plugging point (CFPP) give an indication of the low-temperature behaviour of diesel fuels.
Linear olefin containing streams produced by a Fischer-Tropsch (FT) hydrocarbon synthesis process are currently being used as feed streams for processes in which these olefins are oligomerized to form higher hydrocarbons. The catalyst used for the oligomerization is a shape selective ZSM-5 type zeolite having a medium pore size. The oligomerization products typically contain C1–C24 (gasses+naphtha+diesel) hydrocarbons having internal olefins which are hydrogenated to form paraffins.
The FT feedstock currently used are streams comprising substantially linear, unbranched short chain olefins such as propylene butene, pentene and Hexene derived from a Fischer-Tropsch process. The Iso paraffins produced are heavily branched, contain aromatics and quaternary carbon atoms all of which inhibit biodegradability of the paraffin and results in a low cetane number. Ideally, the paraffin produced should be low in aromatics, naphtha and sulphur, be biodegradable, have a high cetane number (preferably above 40) and a low cloud point without the need for hydroprocessing the paraffin or adding additives to improve the cloud point and/or cetane number after production.
It has been found by the applicant that the above desirable characteristics may be obtained from a feed stream including olefins derived from hydrocarbon producing processes. The diesel fuel produced is useful in environmentally friendly diesel. Kerosene fraction derived along with the diesel fraction can either be used as illuminating paraffin or as a jet fuel blending component in conventional crude or synthetic derived jet fuels or as reactant (especially C10–C13 fraction) in the process to produce LAB (Linear Alkyl Benzene).
The naphtha fraction after hydroprocessing can be routed to a thermal cracker for the production of ethylene and propylene or routed to as is to a catalytic cracker to produce ethylene, propylene and gasoline.
The applicant is also aware that presently oligomerization processes, such as those described above, are carried out on a batchwise basis. Some attempts have been made to make the process semi-continuous by providing a plurality of oligomerization reactors in parallel and in series, typically in a 3 by 3 matrix, thereby permitting the oligomerization reaction to proceed in at least one reactor while the catalyst from other reactors is being regenerated in situ in some of the other reactors which are brought on line once their catalyst has been regenerated.
The reason for the level of complexity appears to be the characteristics of the oligomerization reaction and oligomerization catalyst which leads to fouling and deactivation of the catalyst at a high rate requiring frequent or continuous catalyst regeneration. The fouling/deactivation appears to be in the form of coke or blockage of catalyst pores (active sites) by larger molecules.